Method of manufacturing positive electrode active material for lithium ion battery

ABSTRACT

At least one of an aqueous solution A containing lithium, an aqueous solution B containing iron, manganese, cobalt, or nickel, and an aqueous solution C containing a phosphoric acid includes graphene oxide. The aqueous solution A is dripped into the aqueous solution C, so that a mixed solution E including a precipitate D is prepared. The mixed solution E is dripped into the aqueous solution B, so that a mixed solution G including a precipitate F is prepared. The mixed solution G is subjected to heat treatment in a pressurized atmosphere, so that a mixed solution H is prepared, and the mixed solution H is then filtered. Thus, particles of a compound containing lithium and oxygen which have a small size are obtained.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a positiveelectrode active material for a power storage device and to a powerstorage device.

BACKGROUND ART

With an increase in concern for the environmental issues, power storagedevices such as secondary batteries and electric double layer capacitorsused for power supply for hybrid vehicles and the like have beenactively developed. In particular, a lithium ion battery and a lithiumion capacitor having high energy performance have attracted attention.The lithium ion battery, which is compact but can store a large amountof electricity, has been already mounted on a portable informationterminal such as a mobile phone or a notebook personal computer, and hashelped miniaturization of products.

The secondary battery and the electric double layer capacitor have astructure in which an electrolyte is provided between a positiveelectrode and a negative electrode. It is known that each of thepositive electrode and negative electrode includes a current collectorand an active material provided in contact with the current collector.For example, in a lithium ion battery, a compound capable of occludingand releasing lithium ions is used in electrodes as an active material,and an electrolyte is provided between the electrodes.

Various approaches have been taken to improve the characteristics oflithium ion batteries. Study of positive electrode active materials forlithium ion batteries is one example.

Compounds containing lithium and oxygen, and the like are known as apositive electrode active material of a lithium ion battery (see PatentDocument 1).

In particular, lithium iron phosphate (LiFePO₄) has attracted attentionas a positive electrode active material. Lithium iron phosphate hasadvantages such as inexpensiveness. A lithium ion battery formed usingas a positive electrode active material a compound which involvesFe²⁺/Fe³⁺ oxidation-reduction has the advantages of exhibiting highvoltage (about 3.5 V), having favorable cycle characteristics, havinghigher energy density than a lithium ion battery formed using a compoundsuch as lithium cobalt oxide (LiCoO₂) or lithium nickel oxide (LiNiO₂)as a positive electrode active material because of its theoreticalcapacity of about 170 mAhg⁻¹, and the like.

However, when used as a positive electrode active material of a lithiumion battery, lithium iron phosphate has the problem of difficulty inachieving high output due to its slow lithium diffusion and low electronconductivity. In order to obtain a high-output lithium ion battery byincreasing the area of contact between lithium iron phosphate and anelectrolyte solution, there has been a report of a method for increasinga specific surface area by making lithium iron phosphate crystals intomicroparticles.

For example, it is reported in Non-Patent Document 1 that particles oflithium iron phosphate crystals synthesized by a hydrothermal method ina nitrogen atmosphere have sizes of approximately 300 nm to 500 nm.

REFERENCES

-   [Patent Document 1] Japanese Published Patent Application No.    2008-257894-   [Non-Patent Document 1] Kuwahara et al., “Hydrothermal synthesis of    LiFePO₄ with small particle size and its electrochemical    properties”, Journal of Electroceramics, 2010, Vol. 24, pp. 69-75

DISCLOSURE OF INVENTION

It is an object of one embodiment of the present invention tomanufacture a positive electrode active material including a compoundcontaining lithium and oxygen and having a large specific surface area.It is another object to manufacture a high-output lithium ion batterywith a large area of contact between a positive electrode activematerial and an electrolyte solution.

At least one of aqueous solutions A, B, and C includes graphene oxide.The aqueous solution A is dripped into the aqueous solution C, so that amixed solution E including a precipitate D is prepared. The mixedsolution E is dripped into the aqueous solution B, so that a mixedsolution G including a precipitate F is prepared. The mixed solution Gis subjected to heat treatment in a pressurized atmosphere, so that amixed solution H is prepared, and the mixed solution H is then filtered.Thus, particles of a compound containing lithium and oxygen areobtained. Note that a compound containing lithium and oxygen is referredto as an oxide containing lithium in some cases.

Note that the aqueous solution A includes lithium, the aqueous solutionB includes iron, manganese, cobalt, or nickel, and the aqueous solutionC includes a phosphoric acid.

Note that the obtained particles of the compound containing lithium andoxygen are preferably subjected to cleaning treatment to removeimpurities and then to drying treatment.

The thus manufactured particles of the compound containing lithium andoxygen have a small size owing to the action of graphene oxide. This isattributed to the facts that graphene oxide has a sheet-like shape andgraphene oxide is negatively charged in water. Note that the particlesof the compound containing lithium and oxygen are positively charged inwater.

Since the positively charged particles of the compound containinglithium and oxygen and the negatively charged graphene oxideelectrically attract each other, crystal growth of the particles of thecompound containing lithium and oxygen occurs in the state where theparticles are in contact with graphene oxide. Graphene oxide acts toinhibit crystal growth of the particles of the compound containinglithium and oxygen; thus, the size of the particles of the compoundcontaining lithium and oxygen does not increase easily.

Note that a residue obtained by filtering the mixed solution H includesgraphene oxide besides the target compound containing lithium andoxygen. This graphene oxide may be removed by cleaning treatment orseparation treatment, or may remain.

With the use of graphene oxide and a compound containing lithium andoxygen, a positive electrode active material layer including grapheneoxide among particles of the compound containing lithium and oxygen canbe manufactured. Graphene oxide, which exhibits conductivity dependingon the concentration of oxygen, functions as a conduction auxiliaryagent. In addition, graphene oxide can function as a binder. In otherwords, manufacturing a positive electrode active material layer withgraphene oxide and the compound containing lithium and oxygen which areincluded in the residue can shorten the manufacturing process. Thus, theproductivity in manufacturing positive electrodes including the positiveelectrode active material layer can be increased. In addition, theproductivity in manufacturing lithium ion batteries using the positiveelectrodes can be increased.

Note that reduction treatment may be performed on graphene oxide duringthe manufacturing process of the positive electrode active materiallayer. Graphene oxide with low oxygen concentration or graphene has highconductivity and is suitable for a conduction auxiliary agent.

A specific surface area can be increased when a positive electrode ismanufactured, as described above, using particles of a compoundcontaining lithium and oxygen and serving as a positive electrode activematerial which have a small diameter. Thus, by use of the positiveelectrode, a high-output lithium ion battery can be obtained.

In addition, the productivity in manufacturing lithium ion batteries canbe increased by using graphene oxide included in the residue, as aconduction auxiliary agent and/or a binder of a positive electrodeactive material layer.

A positive electrode active material including a compound containinglithium and oxygen and having a large specific surface area can bemanufactured. In addition, a high-output lithium ion battery with alarge area of contact between a positive electrode active material andan electrolyte solution can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing a method of manufacturing a positiveelectrode active material.

FIG. 2 is a flow chart showing a method of manufacturing a positiveelectrode.

FIG. 3 is a flow chart showing a method of manufacturing a positiveelectrode.

FIG. 4 illustrates a lithium ion battery.

FIGS. 5A and 5B illustrate examples of applications to electronicdevices.

FIG. 6 illustrates an example of application to an electric vehicle.

FIGS. 7A and 7B each illustrate a structure of a negative electrode.

FIG. 8 shows a SEM image of particles of lithium iron phosphate.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details of the present invention canbe modified in various ways. In addition, the present invention shouldnot be construed as being limited to the description in the embodimentsgiven below. Note that, in the description of modes of the presentinvention with reference to the drawings, the same components indifferent diagrams are commonly denoted by the same reference numeral.Note that the same hatch pattern is applied to similar parts, and thesimilar parts are not especially denoted by reference numerals in somecases.

Note that ordinal numbers such as “first” and “second” are used forconvenience and do not denote the order of steps or the stacking orderof layers. In addition, the ordinal numbers in this specification do notdenote any particular names to define the invention.

(Embodiment 1)

In this embodiment, a method of manufacturing a positive electrodeactive material for a lithium ion battery by hydrothermal method whichis one embodiment of the present invention will be described withreference to FIG. 1.

Examples of positive electrode active materials include LiFePO₄, lithiumnickel phosphate (LiNiPO₄), lithium cobalt phosphate (LiCoPO₄), lithiummanganese phosphate (LiMnPO₄), Li₂FeSiO₄, and Li₂MnSiO₄.

For example, the case of manufacturing LiFePO₄ which is a positiveelectrode active material is described. First, lithium hydroxidemonohydrate (LiOH.H₂O), iron(II) chloride tetrahydrate (FeCl₂.4H₂O), andammonium dihydrogen phosphate (NH₄H₂PO₄) which are source materials areweighed so that the molar ratio of LiOH.H₂O to FeCl₂.4H₂O and NH₄H₂PO₄is 2:1:1 (Step S101).

Note that LiOH.H₂O may be replaced with anhydrous lithium hydroxide(LiOH), lithium carbonate (Li₂CO₃), lithium oxide (Li₂O), lithiumnitrate (LiNO₃), lithium dihydrogen phosphate (LiH₂PO₄), lithium acetate(CH₃COOLi), lithium phosphate (Li₃PO₄), or the like.

FeCl₂.4H₂O may be replaced with iron(II) sulfate heptahydrate(FeSO₄.7H₂O), iron(II) phosphate octahydrate (Fe₃(PO₄)₂.8H₂O), iron(II)acetate (Fe(CH₃COO)₂), iron(II) oxalate (FeC₂O₄), iron(II) sulfate(FeSO₄), or the like.

NH₄H₂PO₄ may be replaced with diammonium hydrogen phosphate((NH₄)₂HPO₄), phosphoric acid (H₃PO₄), or the like.

Next, the source materials are each dissolved in water which has beendegassed by nitrogen bubbling in a nitrogen atmosphere. Here, a solutionof LiOH.H₂O in water is referred to as an aqueous solution A, a solutionof FeCl₂.4H₂O in water is referred to as an aqueous solution B, and asolution of NH₄H₂PO₄ in water is referred to as an aqueous solution C.Note that graphene oxide is dispersed in at least one of the aqueoussolutions A, B, and C (Step S102). It is preferable that graphene oxidebe dispersed in the aqueous solution B and the aqueous solution C.Graphene oxide may be dispersed in the aqueous solution by ultrasonictreatment, for example.

Note that in this specification, graphene refers to a one-atom-thicksheet of carbon molecules having holes through which ions can pass andhaving sp² bonds, or a stack of 2 to 100 one-atom-thick sheets thereof.Note that the stack is also referred to as multilayer graphene.

Note that in graphene oxide, part of an end of a graphene sheet isterminated by carboxyl groups (—COOH). Therefore, in an aqueoussolution, hydrogen ions are released from the carboxyl groups andgraphene oxide itself is negatively charged. Note that graphene oxidemay be replaced with graphite oxide.

Next, the aqueous solution A is dripped into the aqueous solution C, sothat a mixed solution E including a precipitate D is prepared (StepS103). Here, the precipitate D is Li₃PO₄.

Then, the mixed solution E is dripped into the aqueous solution B, sothat a mixed solution G including a precipitate F is prepared (StepS104). Here, the precipitate F is a precursor of LiFePO₄.

Next, the mixed solution G is subjected to heat treatment in apressurized atmosphere of 0.1 MPa to 4.0 MPa at a temperature of 100° C.to 250° C. for 1 hour to 24 hours, so that a mixed solution H includinga compound containing lithium and oxygen that is a reaction product isprepared (Step S105). Here, the compound containing lithium and oxygenis LiFePO₄.

Next, the mixed solution H is filtered to give graphene oxide and thecompound containing lithium and oxygen as a residue (Step S106).

Then, the residue is subjected to cleaning treatment and then to dryingtreatment (Step S107). Note that as the cleaning treatment, cleaningwith running pure water may he performed about 10 times, for example. Asthe drying treatment, the residue having been subjected to the cleaningtreatment may be processed in a reduced pressure atmosphere of 3.3×10³Pa at a temperature of 50° C. for 24 hours, for example.

Note that graphene oxide does not react with the compound containinglithium and oxygen. However, graphene oxide attracts the compoundcontaining lithium and oxygen which is positively charged in waterbecause graphene oxide has a sheet-like shape and is negatively chargedin water. Crystal growth of the compound containing lithium and oxygenoccurs in the state where the compound is in contact with grapheneoxide, which inhibits crystal growth of the compound containing lithiumand oxygen. Thus, the size of the resulting particles of the compoundcontaining lithium and oxygen can be small. Specifically, particles ofthe compound containing lithium and oxygen which have a size of 30 nm to400 nm, preferably 30 nm to 250 nm, can be obtained.

The obtained particles of the compound containing lithium and oxygen maybe separated according to the size by using a filter such as a membranefilter.

In this manner, with the use of the aqueous solution including grapheneoxide, the particles of the compound containing lithium and oxygen whichhave a small size can be manufactured.

Note that after the particles of the compound containing lithium andoxygen are obtained, graphene oxide may, but does not necessarily haveto, be removed.

With the use of the thus obtained particles of the compound containinglithium and oxygen which have a small size, a positive electrode activematerial having a large specific surface area can be manufactured.

In addition, when the particles of the compound containing lithium andoxygen which have a small size as described in this embodiment are usedas a positive electrode active material, a high-output lithium ionbattery with a large area of contact between the positive electrodeactive material and an electrolyte solution can be manufactured.

Although the case of manufacturing LiFePO₄ as the compound containinglithium and oxygen is described in this embodiment, the presentinvention is not limited thereto. For example, the present invention canbe applied to manufacturing of LiNiPO₄, LiCoPO₄, LiMnPO₄, Li₂FeSiO₄, orLi₂MnSiO₄ as the compound containing lithium and oxygen.

Specifically, in the case where LiNiPO₄ is manufactured, Li₂CO₃, NiO,and NH₄H₂PO₄ may be used as source materials. In the case where LiCoPO₄is manufactured, Li₂CO₃, CoO, and (NH₄)₂HPO₄ may be used as sourcematerials. In the case where LiMnPO₄ is manufactured, Li₂CO₃, MnCO₃, andNH₄H₂PO₄ may be used as source materials. In the case where Li₂FeSiO₄ ismanufactured, Li₂SiO₃ and FeC₂O₄.2H₂O may be used as source materials.In the case where Li₂MnSiO₄ is manufactured, Li₂SiO₃ and MnC₂O₄ may beused as source materials. Note that the source materials of the positiveelectrode active material given here are mere examples, and the presentinvention is not limited to these materials.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

(Embodiment 2)

In this embodiment, a method of manufacturing a positive electrode of alithium ion battery by using the compound containing lithium and oxygenwhich is manufactured in Embodiment 1 will be described with referenceto FIG. 2.

First, the compound containing lithium and oxygen that is a positiveelectrode active material, a conduction auxiliary agent, and a binderare weighed (Step S201). For example, the proportions of the positiveelectrode active material, the conduction auxiliary agent, and thebinder may range from 80 wt % to 96 wt %, from 2 wt % to 10 wt %, andfrom 2 wt % to 10 wt %, respectively.

Here, as the conduction auxiliary agent, a carbon-based conductionauxiliary agent such as graphene, graphene oxide, graphite, carbonfiber, carbon black, acetylene black, or VGCF (registered trademark), ametal such as copper, nickel, aluminum, or silver, or powder, fiber, orthe like of a mixture thereof may be used. As the binder, apolysaccharide, a thermoplastic resin, a polymer with rubber elasticity,or the like may be used. Examples thereof include graphene, grapheneoxide, starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropylcellulose, regenerated cellulose, diacetyl cellulose, polyvinylchloride, polyvinyl pyrrolidone, polytetrafluoroethylene, polyvinylidenefluoride, polyethylene, polypropylene, ethylene-propylene-diene monomer(EPDM), sulfonated EPDM, styrene-butadiene rubber, butadiene rubber,fluororubber, and polyethylene oxide.

Next, the weighed positive electrode active material, conductionauxiliary agent, and binder are mixed in a solvent, so that a slurry isprepared (Step S202). Here, as the solvent, N-methyl-2-pyrrolidone,lactic acid ester, or the like may be used.

Then, the slurry is applied to a current collector (Step S203). Here, asthe current collector, a highly conductive metal such as aluminum orstainless steel may be used.

Next, the solvent is volatilized from the slurry by drying treatment, sothat a positive electrode active material layer is obtained. A positiveelectrode including the positive electrode active material layer and thecurrent collector is also manufactured (Step S204). Here, the dryingtreatment may be performed in a reduced pressure atmosphere at atemperature of 130° C. for 1 hour, for example.

Note that graphene and graphene oxide function as both the conductionauxiliary agent and the binder. Therefore, in the case where graphene orgraphene oxide is used as the conduction auxiliary agent and the binder,the proportion of the positive electrode active material in the positiveelectrode active material layer can be increased. As a result, thecharge/discharge capacity per volume of a lithium ion battery can beincreased.

In the above manner, the positive electrode of the lithium ion batterycan be manufactured.

With the use of the thus manufactured positive electrode, the area ofcontact between the positive electrode active material and anelectrolyte solution can be increased because particles of the positiveelectrode active material included in the positive electrode have asmall size. Thus, a high-output lithium ion battery can be manufactured.

(Embodiment 3)

In this embodiment, a method of manufacturing a positive electrode byusing the residue including the compound containing lithium and oxygenand graphene oxide which is manufactured in Embodiment 1 will bedescribed with reference to FIG. 3.

As described in Embodiment 2, graphene and graphene oxide function asboth the conduction auxiliary agent and the binder of the positiveelectrode active material layer. Therefore, by utilization of theresidue including the compound containing lithium and oxygen, which is apositive electrode active material, and graphene oxide in Embodiment 1,a positive electrode active material layer can be manufactured withoutseparately preparing a conduction auxiliary agent and/or a binder.

Specifically, first, a slurry in which the compound containing lithiumand oxygen, which is a positive electrode active material, is mixed withgraphene oxide is prepared (Step S301). Note that in the case wheresufficient conductivity or the like cannot be obtained with grapheneoxide included in the residue alone, a smaller amount of conductionauxiliary agent and/or binder than in Embodiment 2 may be mixed.

Next, the slurry is applied to a current collector (Step S302).

Next, the solvent is volatilized from the slurry by drying treatment, sothat a positive electrode active material layer is obtained. A positiveelectrode including the positive electrode active material layer and thecurrent collector is also manufactured (Step S303).

In the above manner, the positive electrode of the lithium ion batterycan be manufactured without separately preparing a conduction auxiliaryagent and/or a binder. Therefore, the number of steps from manufacturinga positive electrode active material to manufacturing a positiveelectrode can be reduced; thus, the productivity in manufacturingpositive electrodes can be improved. In addition, the productivity inmanufacturing lithium ion batteries including the positive electrodescan be increased.

Although the example in which graphene oxide is used as a conductionauxiliary agent and/or a binder is described in this embodiment, thepresent invention is not limited thereto. For example, graphene oxidehaving low oxygen concentration or graphene which is obtained byreducing graphene oxide may be used as a conduction auxiliary agentand/or a binder. Graphene oxide may be reduced before Step S301 or afterStep S303. Graphene oxide having low oxygen concentration or graphenehas high conductivity, and even a small amount of graphene oxide havinglow oxygen concentration or graphene can sufficiently function as aconduction auxiliary agent.

With the use of the thus manufactured positive electrode, the area ofcontact between the positive electrode active material and anelectrolyte solution can be increased because particles of the positiveelectrode active material included in the positive electrode have asmall size. Thus, a high-output lithium ion battery can be manufactured.

(Embodiment 4)

In this embodiment, an example of manufacturing a lithium ion battery,which is a power storage device, by using the positive electrodedescribed in Embodiment 2 or Embodiment 3 will be described. Theschematic structure of the lithium ion battery is illustrated in FIG. 4.

In the lithium ion battery illustrated in FIG. 4, a positive electrode402, a negative electrode 407, and a separator 410 are provided in ahousing 420 which isolates the components from the outside, and thehousing 420 is filled with an electrolyte solution 411. The separator410 is provided between the positive electrode 402 and the negativeelectrode 407.

A positive electrode active material layer 401 is formed in contact witha positive electrode current collector 400. Here, the positive electrodeactive material layer 401 and the positive electrode current collector400 provided with the positive electrode active material layer 401 arecollectively referred to as the positive electrode 402.

On the other hand, a negative electrode active material layer 406 isformed in contact with a negative electrode current collector 405. Here,the negative electrode active material layer 406 and the negativeelectrode current collector 405 provided with the negative electrodeactive material layer 406 are collectively referred to as the negativeelectrode 407.

A first electrode 421 and a second electrode 422 are connected to thepositive electrode current collector 400 and the negative electrodecurrent collector 405, respectively, and charge and discharge areperformed through the first electrode 421 and the second electrode 422.

Moreover, there are certain gaps between the positive electrode activematerial layer 401 and the separator 410 and between the negativeelectrode active material layer 406 and the separator 410. However, thestructure is not limited to this; the positive electrode active materiallayer 401 may be in contact with the separator 410, and the negativeelectrode active material layer 406 may be in contact with the separator410. Further, the lithium ion battery may be rolled into a cylinder withthe separator 410 provided between the positive electrode 402 and thenegative electrode 407.

The positive electrode current collector 400 may have a structuresimilar to that of the current collector described in Embodiment 2 orEmbodiment 3.

For the positive electrode 402, Embodiments 1 to 3 may be referred to.Specifically, the positive electrode 402 is manufactured by forming thepositive electrode active material layer 401 on the positive electrodecurrent collector 400 as follows: the slurry including the positiveelectrode active material described in Embodiment 2 or 3 is dripped andspread thinly by a casting method, and then pressed with a roller pressmachine so that the thickness becomes uniform; after that, reducedpressure drying (under a pressure of 10 Pa or lower) or heat drying (ata temperature of 70° C. to 280° C., preferably 90° C. to 170° C.) isperformed. The positive electrode active material layer 401 may beformed to a thickness with which a crack or detachment (separation) isnot caused, specifically greater than or equal to 20 μm and less than orequal to 100 μm.

As the negative electrode current collector 405, a metal having highconductivity such as copper, stainless steel, iron, or nickel, or thelike can be used.

As the negative electrode active material layer 406, lithium, aluminum,graphite, silicon, germanium, or the like is used. The negativeelectrode active material layer 406 may be formed on the negativeelectrode current collector 405 by a coating method, a sputteringmethod, an evaporation method, or the like. Any of the materials may beused alone as the negative electrode active material layer 406. Thetheoretical lithium ion occlusion capacity of germanium, silicon,lithium, or aluminum is larger than that of graphite. When the lithiumion occlusion capacity is large, sufficient charge/discharge capacitycan be obtained even in a small area. However, silicon increases itsvolume approximately four times its original volume by occluding lithiumions and returns to the original volume by releasing the lithium ions.Thus, charging and discharging of a lithium ion battery may causesilicon itself to become vulnerable or may cause explosion. Therefore, aconfiguration with volume changes taken into consideration ispreferable.

Note that a negative electrode 1100 a or a negative electrode 1100 billustrated in FIG. 7A or 7B may be used as the negative electrode 407.

A negative electrode active material 1108 includes a negative electrodeactive material with a plurality of whiskers which includes a region1107 a and a region 1107 b, and a conductive layer 1113 covering thenegative electrode active material with the plurality of whiskers. Notethat in this specification, the negative electrode active material witha plurality of whiskers refers to a negative electrode active materialincluding a flat region such as the region 1107 a and a regionprojecting from the region 1107 a like a whisker (or like a string or afiber) such as the region 1107 b. Further, in order to clearly show thatthe negative electrode active material 1108 includes a plurality ofwhiskers each of which is a projecting region of the negative electrodeactive material, as illustrated in FIGS. 7A and 7B, the negativeelectrode active material which includes the region 1107 a and theregion 1107 b is referred to as the negative electrode active materialwith a plurality of whiskers. For the conductive layer 1113, graphene orgraphene oxide may be used, for example.

The region 1107 a is provided in contact with the conductive layer 1103,and the region 1107 b projects from the region 1107 a and is providedrandomly. Therefore, the negative electrode active material 1108 has afine surface structure reflecting the shape of the negative electrodeactive material with a plurality of whiskers.

Further, a mixed layer 1105 may be provided in part of or the whole of asurface layer of the conductive layer 1103 by reaction with the negativeelectrode active material 1108 (in particular, the negative electrodeactive material with a plurality of whiskers). Note that the mixed layer1105 also functions as a conductive layer because the mixed layer 1105has conductivity. In the case where the mixed layer 1105 is formed onthe part of the surface layer of the conductive layer 1103, the mixedlayer 1105 and the part of the conductive layer 1103 are provided belowthe negative electrode active material with a plurality of whiskers (inparticular, the region 1107 a) (not illustrated). In the case where themixed layer 1105 is formed on the whole of the surface layer of theconductive layer 1103, the mixed layer 1105 is provided below thenegative electrode active material with a plurality of whiskers (inparticular, the region 1107 a) (see FIGS. 7A and 7B).

Note that the interface between the region 1107 a and the region 1107 bis not clear. Thus, the following surface is defined as the interfacebetween the region 1107 a and the region 1107 b: the lowest surface ofthe negative electrode active material with a plurality of whiskerswhich is parallel to a surface of a substrate 1101 or the conductivelayer 1103.

In the negative electrode active material 1108, the negative electrodeactive material with a plurality of whiskers preferably includes a core1109 which has a crystalline structure and an outer shell 1111 which hasan amorphous structure. The outer shell 1111 having the amorphousstructure is resistant to changes in volume due to occlusion and releaseof lithium ions. The core 1109 having the crystalline structure has ahigh conductivity and has a feature of high-speed occlusion and releaseof lithium ions per mass. Therefore, by using the negative electrode1100 a or the negative electrode 1100 b including the negative electrodeactive material with a plurality of whiskers including the core 1109 andthe outer shell 1111, high-speed charging and discharging can beachieved, and a lithium ion battery with excellent charge/dischargecapacity and cycle characteristics can be manufactured.

Note that the core 1109 is not limited to the core which is in contactwith the conductive layer 1103 such as a core 1109 b, and may be thecore which extends from front to back of the drawing such as a core 1109c and the core which is localized such as a core 1109 a. That is, thecore 1109 a, the core 1109 b, and the core 1109 c are collectivelyreferred to as the core 1109. Further, an outer shell 1111 a, an outershell 1111 b, and an outer shell 1111 c are collectively referred to asthe outer shell 1111.

The plurality of whiskers of the negative electrode active material inthe region 1107 b may have a columnar shape, a conical or pyramidalshape, or a needle-like shape. The plurality of whiskers of the negativeelectrode active material may be curved. The plurality of whiskers ofthe negative electrode active material may have a round end.

The plurality of whiskers of the negative electrode active material inthe region 1107 b may have different longitudinal directions. When theplurality of whiskers of the negative electrode active material havedifferent longitudinal directions, in FIGS. 7A and 7B, a transversecross-sectional shape of a whisker of the negative electrode activematerial (the cross-sectional shape of a portion including the core 1109c and the outer shell 1111 c) is shown as well as a longitudinalcross-sectional shape of a whisker of the negative electrode activematerial (the cross-sectional shape of a portion including the core 1109b and the outer shell 1111 b). In a transverse cross section of theplurality of whiskers of the negative electrode active material, thecore 1109 is observed (or not observed) in the plurality of whiskers ofthe negative electrode active material in some cases depending on aposition. Further, the transverse cross section of a whisker of thenegative electrode active material is circular when the whisker of thenegative electrode active material has a cylindrical or conical shape,and is polygonal when the whisker of the negative electrode activematerial has a prismatic or pyramidal shape. It is preferable that thelongitudinal directions of the plurality of whiskers of the negativeelectrode active material be not uniform because in that case, one ofthe whiskers of the negative electrode active material is likely to beentangled with the other, so that detachment (or separation) of theplurality of whiskers of the negative electrode active material does noteasily occur.

Note that the direction in which each of the plurality of whiskers ofthe negative electrode active material is extended from the region 1107a is called the longitudinal direction, and the cross-sectional shapethereof cut along the longitudinal direction is called a longitudinalcross-sectional shape. In addition, a cross-sectional shape of each ofthe plurality of whiskers of the negative electrode active material cutalong a surface perpendicular to or substantially perpendicular to thelongitudinal direction is called a transverse cross-sectional shape.

The maximum diameter of the core 1109 in the transverse cross sectionmay be greater than or equal to 0.2 μm and less than or equal to 3 μm,preferably greater than or equal to 0.5 μm and less than or equal to 2μm.

The length of the core 1109 is not particularly limited but may begreater than or equal to 0.5 μm and less than or equal to 1000 μm,preferably greater than or equal to 2.5 μm and less than or equal to 100μm.

In the region 1107 b, the maximum diameter of the plurality of whiskersof the negative electrode active material in the transverse crosssection is greater than or equal to 0.2 μm and less than or equal to 10μm, preferably greater than or equal to 1 μm and less than or equal to 5μm. The length of the plurality of whiskers of the negative electrodeactive material is greater than or equal to 3 μm and less than or equalto 1000 μm, preferably greater than or equal to 6 μm and less than orequal to 200 μm.

Note that the “length” of the core 1109 or the outer shell 1111 refersto the distance between the top of the core 1109 or the outer shell 1111and the region 1107 a, in the direction along an axis passing throughthe center of the top (or the top surface) of the whisker of thenegative electrode active material.

Further, the structure of the negative electrode active material with aplurality of whiskers is not limited to the above structure; the wholeof the region 1107 a and the region 1107 b may have a crystallinestructure, or the whole of the region 1107 a and the region 1107 b mayhave an amorphous structure.

In the negative electrode 1100 a illustrated in FIG. 7A, part of theregion 1107 a (a region other than the region where the conductive layer1103 is in contact with the core 1109) has an amorphous structure likethe outer shell 1111. Further, the region 1107 a may include acrystalline structure. Furthermore, the region 1107 a may include one ormore of the elements constituting either the conductive layer 1103 orthe mixed layer 1105 or both.

In the negative electrode 1100 b illustrated in FIG. 7B, a region of theregion 1107 a on the side in contact with the conductive layer 1103 hasa crystalline structure like the core 1109. Further, the region 1107 amay include an amorphous structure. Furthermore, the region 1107 a mayinclude one or more of the elements constituting either the conductivelayer 1103 or the mixed layer 1105 or both.

For example, the negative electrode 1100 a has a higher adhesion betweenthe conductive layer 1103 and the region 1107 a than the negativeelectrode 1100 b. This is because an amorphous structure is moreadaptable to the surface of the conductive layer 1103, on which theregion 1107 a is formed. In other words, an amorphous structure is morelikely to be formed so as to be compatible with the surface of theconductive layer 1103. In addition, an amorphous structure is resistantto volume changes due to occlusion and release of lithium ions; thus,the negative electrode 1100 a (especially the negative electrode activematerial with a plurality of whiskers) can be prevented from beingdetached due to repetitive charging and discharging, and a lithium ionbattery with good cycle characteristics can be manufactured.

In the negative electrode 1100 b, a highly conductive crystallinestructure is in contact with a wider area of the conductive layer 1103than in the negative electrode 1100 a. Therefore, the negative electrode1100 b as a whole has high conductivity. Accordingly, a lithium ionbattery capable of being charged and discharged at high speed and havinghigh charge/discharge capacity can be manufactured.

The negative electrode active material with a plurality of whiskers canbe formed by a low pressure chemical vapor deposition (LPCVD) method.Here, the negative electrode active material with a plurality ofwhiskers is formed at a temperature higher than 400° C. and lower thanor equal to a temperature which an LPCVD apparatus, the substrate 1101,and the conductive layer 1103 can withstand, and preferably higher thanor equal to 500° C. and lower than 580° C.

In the case where the negative electrode active material with aplurality of whiskers is formed, as a source gas, a deposition gascontaining silicon is used. Specifically, SiH₄, SiF₄, SiCl₄, Si₂Cl₆, orthe like may be used as the deposition gas containing silicon. Note thatone or more kinds of a hydrogen gas and rare gases such as helium, neon,argon, and xenon may be contained in the source gas.

As the electrolyte, an electrolyte solution that is an electrolyte in aliquid state, or a solid electrolyte that is an electrolyte in a solidstate may be used. The electrolyte solution contains lithium ions and isresponsible for electrical conduction.

The electrolyte solution 411 includes, for example, a solvent and alithium salt dissolved in the solvent. Examples of the lithium saltinclude LiCl, LiF, LiClO₄, LiBF₄, LiAsF₆, LiPF₆, and Li(C₂F SO₂)₂N.

Examples of the solvent of the electrolyte solution 411 include cycliccarbonates (such as ethylene carbonate (EC), propylene carbonate (PC),butylene carbonate (BC), and vinylene carbonate (VC)), acycliccarbonates (such as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), isobutylmethyl carbonate, and dipropyl carbonate (DPC)), aliphatic carboxylicacid esters (such as methyl formate, methyl acetate, methyl propionate,and ethyl propionate), acyclic ethers (such as γ-lactones such asγ-butyrolactone, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),and ethoxymethoxy ethane (EME)), cyclic ethers (such as tetrahydrofuranand 2-methyltetrahydrofuran), cyclic sulfones (such as sulfolane), alkylphosphate esters (such as dimethylsulfoxide, 1,3-dioxolane, trimethylphosphate, triethyl phosphate, and trioctyl phosphate), and fluoridesthereof. These materials can be used either alone or in combination asthe solvent of the electrolyte solution 411.

As the separator 410, paper, nonwoven fabric, a glass fiber, a syntheticfiber such as nylon (polyamide), vinylon (a polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane, or the like maybe used. Note that the separator 410 is not soluble in the electrolytesolution 411 described above.

More specific examples of materials of the separator 410 arehigh-molecular compounds based on fluorine-based polymer, polyether suchas polyethylene oxide and polypropylene oxide, polyolefin such aspolyethylene and polypropylene, polyacrylonitrile, polyvinylidenechloride, polymethyl methacrylate, polymethylacrylate, polyvinylalcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,polyethyleneimine, polybutadiene, polystyrene, polyisoprene, andpolyurethane, derivatives thereof, cellulose, paper, and nonwovenfabric, which may be used either alone or in combination.

When the lithium ion battery described above is charged, a positiveelectrode terminal is connected to the first electrode 421 and anegative electrode terminal is connected to the second electrode 422. Anelectron is taken away from the positive electrode 402 through the firstelectrode 421 and transferred to the negative electrode 407 through thesecond electrode 422. In addition, a lithium ion is eluted from thepositive electrode active material in the positive electrode activematerial layer 401 of the positive electrode 402, reaches the negativeelectrode 407 through the separator 410, and is taken into the negativeelectrode active material in the negative electrode active materiallayer 406. The lithium ion and the electron are combined in this regionand are occluded in the negative electrode active material layer 406. Atthe same time, in the positive electrode active material layer 401, anelectron is released outside from the positive electrode activematerial, and an oxidation reaction of a transition metal (such as iron,manganese, cobalt, or nickel) contained in the positive electrode activematerial occurs.

At the time of discharge of the lithium ion battery, in the negativeelectrode 407, the negative electrode active material layer 406 releasesan lithium ion, and an electron is transferred to the second electrode422. The lithium ion passes through the separator 410, reaches thepositive electrode active material layer 401, and is taken into thepositive electrode active material in the positive electrode activematerial layer 401. At that time, an electron from the negativeelectrode 407 also reaches the positive electrode 402, and a reductionreaction of the transition metal contained in the positive electrodeactive material occurs.

With the use of the positive electrode described in Embodiment 2 orEmbodiment 3, a high-output lithium ion battery can be manufactured.

In the case where graphene or graphene oxide is used as a conductionauxiliary agent of a positive electrode active material layer, theamount of the conduction auxiliary agent needed to improve conductivitycan be small. Thus, the volume of the positive electrode active materiallayer can be small, and a positive electrode capable of easily occludingand releasing lithium ions can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

(Embodiment 5)

In this embodiment, applications of the lithium ion battery described inEmbodiment 4 will be described.

The lithium ion battery described in Embodiment 4 can be used inelectronic devices, e.g., cameras such as digital cameras or videocameras, digital photo frames, portable information terminals (alsoreferred to as cellular phones, cellular phone devices, tablet PCs, orthe like), portable game machines, audio reproducing devices, and thelike. Further, the lithium ion battery can be used in electricpropulsion vehicles such as electric vehicles, hybrid electric vehicles,train vehicles, maintenance vehicles, carts, wheelchairs, or bicycles.

FIG. 5A illustrates an example of a portable information terminal. In aportable information terminal 510, a display portion 512 is incorporatedin a housing 511. The housing 511 is provided with an operation button513, an operation button 517, an external connection port 514, a speaker515, a microphone 516, and the like.

FIG. 5B illustrates another portable information terminal which isdifferent from the portable information terminal 510. A portableinformation terminal 530 includes two housings, a first housing 531 anda second housing 533, which are combined with each other with a hinge532. The first housing 531 and the second housing 533 can be opened andclosed with the hinge 532 used as an axis. A first display portion 535and a second display portion 537 are incorporated in the first housing531 and the second housing 533, respectively. Note that the firstdisplay portion 535 and/or the second display portion 537 may be touchpanels. In addition, the second housing 533 is provided with anoperation button 539, a power switch 543, a speaker 541, and the like.

FIG. 6 illustrates an example of an electric vehicle. An electricvehicle 550 is equipped with a lithium ion battery 551. The output ofpower of the lithium ion battery 551 is controlled by a control circuit553 and the power is supplied to a driving device 557. The controlcircuit 553 is controlled by a computer 555.

The driving device 557 includes an electric motor (a DC motor or an ACmotor), and, if necessary, an internal-combustion engine. In the casewhere the internal-combustion engine is incorporated, theinternal-combustion engine and the electric motor are combined. Thecomputer 555 outputs a control signal to the control circuit 553 on thebasis of data of order (such as acceleration or stop) by a driver of theelectric vehicle 550 or data of driving environment (such as an upgradeor a downgrade). The control circuit 553 adjusts the electric energysupplied from the lithium ion battery 551 in accordance with the controlsignal of the computer 555 to control the output of the driving device557. In the case where the AC motor is mounted, an inverter whichconverts direct current into alternate current is incorporated.

The lithium ion battery 551 can be charged by power supply from theoutside.

Note that in the case where the electric propulsion vehicle is a trainvehicle, the train vehicle can be charged by power supply from anoverhead cable or a conductor rail.

EXAMPLE 1

In this example, an example of manufacturing LiFePO₄ which is a positiveelectrode active material and has a small particle size will bedescribed.

First, LiOH.H₂O, FeCl₂.4H₂O, and NH₄H₂PO₄ which were source materialswere weighed so that the molar ratio of LiOH.H₂O to FeCl₂.4H₂O andNH₄H₂PO₄ was 2:1:1. Here, the source materials were weighed so that Fewas 0.2 M with respect to 100 ml of water.

Next, the source materials were each dissolved in water which had beendegassed by nitrogen bubbling in a nitrogen atmosphere. Here, a solutionof LiOH.H₂H in 30 ml of water is referred to as an aqueous solution A, asolution of FeCl₂.4H₂O in 30 ml of water is referred to as an aqueoussolution B, and a solution of NH₄H₂PO₄ in 30 ml of water is referred toas an aqueous solution C. Note that 30 mg of graphene oxide powder wasadded to the aqueous solution B. In addition, 60 mg of graphene oxidepowder was added to the aqueous solution C. After that, graphene oxidewas dispersed in the aqueous solution B and the aqueous solution C byultrasonic treatment.

Next, the aqueous solution A was dripped into the aqueous solution C, sothat a mixed solution including Li₃PO₄ as a precipitate was obtained.

Next, the obtained mixed solution was dripped into the aqueous solutionB, so that a mixed solution including a precursor of LiFePO₄ as aprecipitate was obtained. Next, the obtained mixed solution was degassedby nitrogen bubbling, and 10 ml of pure water degassed similarly bynitrogen bubbling was added to the mixed solution to give 100 ml of amixed solution.

Next, the obtained mixed solution was subjected to heat treatment in apressurized atmosphere of 0.4 MPa at a temperature of 150° C. for 15hours, so that a mixed solution including LiFePO₄ which was a reactionproduct was obtained.

Next, the obtained mixed solution was filtered to give LiFePO₄.

Next, LiFePO₄ was subjected to cleaning with running pure water 10times, and dried in a reduced pressure atmosphere of 3.3×10³ Pa at atemperature of 50° C. for 24 hours.

LiFePO₄ obtained in the above manner was observed with a scanningelectron microscope (SEM) (see FIG. 8). As a result, substantiallyrectangular-solid crystal particles of LiFePO₄ roughly with a size of(50 nm to 200 nm, 50 nm to 200 nm, 50 nm to 400 nm) were observed.

It can be seen that LiFePO₄ with a small particle size can be obtainedby the method described in this example.

EXPLANATION OF REFERENCE

S101: step, S102: step, S103: step, S104: step, S105: step, S106: step,S107: step, S201: step, S202: step, S203: step, S204: step, S301: step,S302: step, S303: step, 400: positive electrode current collector, 401:positive electrode active material layer, 402: positive electrode, 405:positive electrode current collector, 406: negative electrode activematerial layer, 407: negative electrode, 410: separator, 411:electrolyte solution, 420: housing, 421: first electrode, 422: secondelectrode, 510: portable information terminal, 511: housing, 512:display portion, 513: operation button, 514: external connection port,515: speaker, 516: microphone, 517: operation button, 530: portableinformation terminal, 531: first housing, 532: hinge, 533: secondhousing, 535: first display portion, 537: second display portion, 539:operation button, 541: speaker, 543: power switch, 550: electricvehicle, 551: lithium ion battery, 553: control circuit, 555: computer,557: driving device, 1100 a: negative electrode, 1100 b: negativeelectrode, 1101: substrate, 1103: conductive layer, 1105: mixed layer,1107 a: region, 1107 b: region, 1108: negative electrode activematerial, 1109: core, 1109 a: core, 1109 b: core, 1109 c: core, 1111:outer shell, 1111 a: outer shell, 1111 b: outer shell, 1111 c: outershell, and 1113: conductive layer.

This application is based on Japanese Patent Application serial no.2011-186340 filed with Japan Patent Office on Aug. 29, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method of manufacturing an active materialparticle, comprising the steps of: preparing a first mixed solutioncomprising lithium and oxygen; preparing a second mixed solutioncomprising a metal and oxygen; and obtaining a particle of a compoundcomprising lithium, the metal and oxygen by mixing together the firstmixed solution and the second mixed solution, wherein each of the firstmixed solution and the second mixed solution comprises graphene oxide.2. The method of manufacturing an active material particle according toclaim 1, wherein the metal is one of iron, manganese, cobalt and nickel.3. The method of manufacturing an active material particle according toclaim 1, wherein the first mixed solution comprises a phosphoric acid.4. The method of manufacturing an active material particle according toclaim 1, wherein the first mixed solution comprises silicon.
 5. Themethod of manufacturing an active material particle according to claim1, wherein the particle is obtained by subjecting the second mixedsolution to a heat treatment in an atmosphere of 0.1 MPa to 4.0 MPa. 6.The method of manufacturing an active material particle according toclaim 1, wherein the compound has a size of 30 nm to 250 nm.
 7. Themethod of manufacturing an active material particle according to claim1, wherein the particle comprises iron.
 8. The method of manufacturingan active material particle according to claim 1, wherein the particlecomprises manganese.
 9. The method of manufacturing an active materialparticle according to claim 1, wherein the particle comprises cobalt.10. The method of manufacturing an active material particle according toclaim 1, wherein the particle comprises nickel.
 11. The method ofmanufacturing an active material particle according to claim 1, furthercomprising the step of: performing a heat treatment of the particle. 12.The method of manufacturing an active material particle according toclaim 11, wherein the heat treatment is a drying treatment.
 13. Themethod of manufacturing an active material particle according to claim3, wherein the particle comprises any one of LiNiPO₄, LiCoPO₄, LiMnPO₄and LiFePO₄.
 14. The method of manufacturing an active material particleaccording to claim 4, wherein the particle comprises at least any one ofLi₂MnSiO₄ and Li₂FeSiO₄.
 15. The method of manufacturing an activematerial particle according to claim 5, wherein the particle comprisesgraphene oxide.
 16. A method of manufacturing an active materialparticle, comprising the steps of: preparing a first mixed solutioncomprising lithium and oxygen; preparing a second mixed solutioncomprising a metal and oxygen; and obtaining a particle of a compoundcomprising lithium, the metal and oxygen by mixing together the firstmixed solution and the second mixed solution, wherein each of the firstmixed solution and the second mixed solution comprises graphene oxide,and wherein the particle is obtained by subjecting the second mixedsolution to a heat treatment in an atmosphere of 0.1 MPa to 4.0 MPa. 17.A method of manufacturing an active material particle, comprising thesteps of: preparing a first mixed solution comprising lithium andoxygen; preparing a second mixed solution comprising a metal and oxygen;and obtaining a particle of a compound comprising lithium, the metal andoxygen by mixing together the first mixed solution and the second mixedsolution, wherein each of the first mixed solution and the second mixedsolution comprises graphene oxide, wherein the first mixed solutioncomprises a phosphoric acid, wherein the particle comprises any one ofLiNiPO₄, LiCoPO₄, LiMnPO₄ and LiFePO₄, and wherein the particle isobtained by subjecting the second mixed solution to a heat treatment inan atmosphere of 0.1 MPa to 4.0 MPa.
 18. A method of manufacturing anactive material particle, comprising the steps of: preparing a firstmixed solution comprising lithium and oxygen; preparing a second mixedsolution comprising a metal and oxygen; and obtaining a particle of acompound comprising lithium, the metal and oxygen by mixing together thefirst mixed solution and the second mixed solution, wherein each of thefirst mixed solution and the second mixed solution comprises grapheneoxide, wherein the first mixed solution comprises silicon, wherein theparticle comprises at least any one of Li₂MnSiO₄ and Li₂FeSiO₄, andwherein the particle is obtained by subjecting the second mixed solutionto a heat treatment in an atmosphere of 0.1 MPa to 4.0 MPa.